In order to understand diseases of the blood, such as leukemias, lymphomas and autoimmune diseases, it is essential to understand the full complement of gene expression that occurs the hematopoietic stem cells (HSCs) that give rise to the blood lineage. Although in the adult, HSCs are found primarily in the bone marrow, in the embryo they can be traced to the fetal liver, the aorta-gonad-mesonephros and the endothelium of the placenta. In 2011 we published a paper (Dev Dynamics 240:2290) in which we characterized the embryonic lineage during mouse development that is marked by the expression of the Tbx4 gene, which encodes a transcription factor of the T-box class. This paper also characterized a useful mouse line, called Tbx4-Cre, that we have since used, in a collaborative effort, to demonstrate that Roundabout receptors are critical for foregut separation from the body wall during embryogenesis (Dev Cell 24: 52) Our analysis of the Tbx4 lineage in the extraembryonic tissue inspired us to speculate that Tbx4 expression marked early cells into two compartments, each giving rise to an endothelium, only one of which is capable of generating HSCs (hemogenic endothelium). If this hypothesis is correct, Tbx4 expression may be one of the earliest known markers useful in isolating and purifying cells that will generate hemogenic endothelium. Therefore, this year we have embarked on experiments to test this hypothesis and have generated data that demonstrates that Tbx4 either is a superior to other commonly used markers of hemogenic endothelium or, when combined with these other markers, vastly improves our ability to isolate this crucial cellular subtype. We are now embarking on experiments to use this new approach to isolate and characterize the genes that are expressed in these cells. Such data will allow us to understand the biology of the HSC.